This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The major goals of this project have been to (1) determine whether sequence variants within gp120 are selected during transmission;(2) determine whether the selection of viral variants reflects the population of virus present in the genital fluids of the donor or a biological restriction of the transmission process;and (3) determine the frequency, kinetics, and the virologic and immunologic ramifications of HIV superinfection in both partners following early infection. The results of these studies are enhancing our understanding of the heterosexual transmission process and yield novel information that is critical to the design and testing of globally effective vaccine candidates. We are analyzing HIV-1 virus populations, isolated from both the donor and recipient immediately following a transmission event, in unique cohorts of discordant couples in Rwanda and Zambia. These studies have provided a unique opportunity to investigate both the virologic determinants of heterosexual transmission as well as the frequency and implications of HIV superinfection using samples from large, well-characterized discordant couple cohorts that represent two predominant clades (A and C) of HIV-1. Recent studies confirm that a severe genetic bottleneck occurs during transmission with a majority of seroconverting partners being infected by a single genetic variant from their spouse, although inflammatory genital infections can increase the number of variants establishing infection. Newly transmitted viruses exhibit a dependence on high levels of both CCR5 and CD4 for entry, and infect macrophages with low efficiency. Despite compartmentalization of virus variants in the genital tract of both men and women, the viruses that establish infection in the spouse appear to be distinct from these enriched populations [unreadable]consistent with the bottleneck to transmission occurring at the mucosal surface. Finally, superinfection was detected in 5 of 22 couples where infection occurred outside the marriage and the viruses in each partner are genetically different.